1. Field of the Invention
This invention relates generally to earth-boring drill bits and particularly to improved cutting structures for such bits.
2. Background of the Art
In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters or cones mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The rolling cutters are provided with cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight of the drillstring. The cuttings from the bottom sidewalls of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drillstring.
One type of cutting element in widespread use is a tungsten carbide insert which is interference pressed into an aperture in the cutter body. Tungsten carbide is metal which is harder than the steel body of the cutter and has a cylindrical portion and a cutting tip portion. The cutting tip portion is formed in various configurations, such as chisel, hemispherical or conical, depending on the type of formation to be drilled. Some of the inserts have very aggressive cutting structure designs and carbide grades that allow the bits to drill in both soft and medium formations with the same bit.
Another type of rolling cutter earth-boring bit is commonly known as a xe2x80x9csteel toothxe2x80x9d or xe2x80x9cmilled toothxe2x80x9d bit. Typically these bits are for penetration into relatively soft geological formations of the earth. The strength and fracture toughness of the steel teeth permits the use of relatively long teeth, which enables the aggressive gouging and scraping actions that are advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel-tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks. Consequently, it has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called xe2x80x9chardfacingxe2x80x9d over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide, dispersed in a steel matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced and applying the hard particles to the matrix to form a uniform dispersion of hard particle in the matrix.
Typical hardfacing deposits are welded over a steel tooth that has been machined similar to the desired final shape. The hardfacing materials do not have a tendency to heat crack, which helps counteract the occurrence of frictional heat cracks associated with carbide inserts. The hardfacing is much harder than the steel tooth inserts, therefore the hardfacing on the surface of steel teeth makes the teeth more resistant to wear.
Developments in hardfacing materials and welding skill have improved the overall quality of the hardfacing deposits, which allows for thicker deposits to be welded onto the teeth, which are usually smaller to accommodate the addition of hardfacing materials. However, the geometry of the tooth profile can vary considerably depending on the skill of the welder, the geometry of the tooth that the hardfacing is being applied to, and the desired geometry of the desired tooth after the hardfacing is applied. These variables have produced cutting elements which were not uniform throughout their respective rows, and which were only capable of having the final shape after hardfacing. In the xe2x80x9cas-weldedxe2x80x9d state, the cutting efficiency of the bit was not optimal because the cutting elements were not uniform within their respective cutting rows. Furthermore, cutting efficiency was not optimal because the smoothness of the hardfacing varied depending on welder skill.
In the prior art, hardfacing on the gauge surface of the cone is ground smooth so that the bit remains the desired diameter. However, the hardfacing on the leading and trailing flanks of the teeth is not ground.
An earth-boring bit has a bit body and at least one cantilevered bearing shaft depending inwardly and downwardly from the bit body. A cutter is mounted for rotation on each bearing shaft wherein each cutter includes a plurality of cutting elements. The cutting elements are arranged in circumferential rows on the cutter and at least some of the cutter elements comprise teeth. At least some of the teeth have a hardfacing composition of carbide particles dispersed in a metallic matrix, which has at least one smooth ground flank.
The purpose of this invention is to allow for the mechanical shaping of the welded tooth deposits into more useable cutting/wear elements. This would allow for the shaping of several different geometries from typical hardfacing deposits. This also allows for differences in geometry of teeth on the same row or on different rows or in between the rows, or anywhere on the immediate cone shell. Shaped hardfacing cutting/wear elements can be used on a variety of cutting materials including steel teeth, tungsten carbide teeth bits, diamond bits, or other downhole tools. The shaping of the cutting/wear element could be accomplished by grinding, plunge electrical-discharge machining (EDM), wire EDM, laser machining, or by any other method capable of shaping hardfacing after it is applied.